Electrodeposition of thermoelectric Bi2Te3 thin films with added surfactant

Current Applied Physics 15 (2015) 261e264

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Electrodeposition of thermoelectric Bi2Te3 thin films with added surfactant Youngsup Song a, In-Joon Yoo a, Na-Ri Heo a, b, Dong Chan Lim a, Dongyun Lee c, Joo Yul Lee a, Kyu Hwan Lee a, Kwang-Ho Kim b, Jae-Hong Lim a, * a b c

Electrochemistry Department, Korea Institute of Materials Science, Changwon 641-010, South Korea Department of Materials Science and Engineering, Pusan National University, Busan 609-735, South Korea Department of Nanofusion Technology, Pusan National University, Busan 609-735, South Korea

a r t i c l e i n f o

a b s t r a c t

Article history: Received 5 August 2014 Received in revised form 13 November 2014 Accepted 2 December 2014 Available online 20 December 2014

Bismuth telluride (Bi2Te3) thin films were electrodeposited at room temperature from nitric baths in the presence of a surfactant, cetyltrimethylammonium bromide (CTAB). Nearly stoichiometric Bi2Te3 thin films were obtained from electrolytes containing 7.5 mM Bi(NO3)3. The surface morphology and mechanical properties of the electrodeposited thin film were improved by the addition of CTAB to the electrolyte, while the electrical and thermoelectric properties were preserved. Post-deposition annealing in a reducing environment did not improve the electrical and thermoelectric properties, possibly because the change in the microstructure of the Bi2Te3 thin film was too small. © 2014 Elsevier B.V. All rights reserved.

Keywords: BixTey Electrodeposition Power factor Thermoelectrics Mechanical properties

1. Introduction Bismuth telluride (Bi2Te3)-containing alloys can be used for various applications including solar cells, thermoelectric (TE) devices, and phase-change devices [1e3]. In particular, TE energy converters that make use of such alloys have attracted much attention, as they exhibit numerous interesting features such as solid-state operation, zero emissions, vast scalability, low maintenance, and long operating lifetimes [4e7]. The efficiency of a TE material is directly related to a dimensionless figure of merit, zT, which is defined as zT ¼ S2sT/k, where S is the Seebeck coefficient of the material in question, s and k are its respective electrical and thermal conductivity, and T is absolute temperature. The variables S, s, and k for conventional bulk-crystalline systems are interrelated in such a way that it is very difficult to control them independently to increase zT. This is because an increase in S usually results in a decrease in s, which produces a decrease in the electronic contribution to k according to the WiedemanneFranz law [8]. The introduction of materials with low dimensionality and

* Corresponding author. E-mail address: [email protected] (J.-H. Lim). http://dx.doi.org/10.1016/j.cap.2014.12.004 1567-1739/© 2014 Elsevier B.V. All rights reserved.

many interfacesdthe latter of which scatter phonons more effectively than electrons or serve to filter out low-energy electrons at interfacial energy barriersdpermits the development of nanostructured materials with enhanced energy-conversion efficiencies. The incorporation of these heterogeneous nanostructures within materials, such as the introduction of Bi2Te3 to alloys designed for TE devices, can enable one to manipulate the variables S, s, and k somewhat independently. A number of well-established techniques such as metal-organic chemical vapor deposition (MOCVD), physical vapor deposition (PVD), molecular beam epitaxy (MBE), and electrodeposition have been used to fabricate high-efficiency thermoelectric thin films [9e13]. Compared to the other methods, electrodeposition is one the most cost-effective techniques for the fabrication of nanostructured materials. However, electrodeposited films have crystal structures, thermoelectric characteristics, and morphologies that are poorer than those of their corresponding bulk materials. This results in weak mechanical properties, the opposite of which are essential for the fabrication of thermoelectric devices [14,15]. In this study, we investigated the effects of surfactant addition on electrodeposited BixTey thin films in terms of morphology as well as their electrical, thermoelectric, and mechanical properties. Prior to potentiostatic deposition, linear sweep voltammograms

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(LSVs) were acquired to investigate the effects of the surfactant used. The electrodeposited BixTey thin films prepared with CTAB (cetyltrimethylammonium bromide) were examined methodically to correlate their morphologies with electrical, thermoelectric, and mechanical properties. 2. Experimental details Prior to potentiostatic electrodeposition of the Bi2Te3 films, LSVs were taken using a standard three-electrode cell. The substrate, which consisted of an 80-nm Au layer on top of a 20-nm Ni layer deposited on a Si wafer (Au/Ni/Si), served as the working electrode in electrolyte solutions of 10 mM TeO2 and 1.5 M HNO3, wherein the Bi(NO3)3 concentration was varied from 5 to 15 mM. The electrolytes were prepared by dissolving TeO2 powder in concentrated nitric acid, followed by dissolution of Bi(NO3)3 powder in a separate container. Once the chemicals were completely dissolved, they were mixed together and then diluted with deionized water until a desired concentration was attained. The effects of the surfactant were studied by adding 0.75 mM CTAB to this solution. All

experiments were conducted at room temperature with a fixed magnetic agitation rate of 300 rpm and a scan rate of 1 mV s1. The annealing temperature was varied between 250 and 300  C for 30 min under a nitrogen atmosphere with 5% hydrogen. The morphologies and compositions of the Bi2Te3 films were analyzed using a scanning electron microscope (SEM; JSM-5800, JEOL, Japan) at an accelerating voltage of 20 kV. An energydispersive spectrometer (EDS) was equipped to the SEM. To confirm the crystallinity, X-ray diffraction (XRD) patterns were measured by an X-ray diffractometer (X'Pert-PRO, PANalytical, The Netherlands) using Cu Ka radiation at a voltage of 40 kV and a current of 30 mA. To measure the thermoelectric and electrical properties, the films were detached from the substrate using Torr Seal epoxy (Varian Vacuum Products, USA). The electrical properties were measured with a custom-made Hall-effect measurement device in the van der Pauw configuration, using the four-point probe method. The Seebeck coefficient was measured using laboratory-built Seebeck-measurement equipment; the in-plane Seebeck coefficient was determined from plots of the measured Seebeck voltage as a function of the temperature difference (<2  C) with a two-probe distance of about 5 mm across the specimen (S ¼ DV/DT). The mechanical properties of the Bi2Te3 thin films were also investigated using a nanoindentation system (G-200, Agilent Technologies, USA). These measurements were made using a threesided pyramidal Berkovich diamond indenter tip, using the continuous-stiffness measurement (CSM) mode. The maximum indentation depth was 360 nm, about 20% of the thickness of the films. The indentations were made at a constant strain rate of 0.05 S1. The hardness and elastic modulus values of the films were averaged over 12 indentation points in each specimen. The average hardness and modulus values were calculated over a depth range of 150e250 nm. 3. Results and discussion The LSVs were taken to evaluate the electrochemical reaction during electrodeposition of BixTey thin films on Au/Ni/Si substrates. In the absence of CTAB, the first reduction peak was observed at approximately 0.1 V vs. the saturated calomel electrode (SCE), as shown in Fig. 1(a). This reduction peak corresponds to the reduction of HTeOþ 2 ions compared to elemental Te atoms, which was followed by the underpotential deposition of Bi3þ onto Te at approximately 0.0 V vs. SCE. Owing to the negative Gibbs free energy of Bi2Te3, Bi could only be deposited for Bi2Te3 formation by underpotential deposition. Thus, the deposition of Bi2Te3 was limited by the deposition of Te. The reduction reaction can be expressed as follows [16]:

3HTeO2þ þ 2Bi3þ þ 9Hþ þ 18e ¼ Bi2 Te3 þ 6H2 O

Fig. 1. (a) LSVs of BixTy electrodeposited on Au/Ni/Si substrates with and without the surfactant CTAB. The scan rate was fixed at 1 mV s1. (b) Ratio of Te to Bi in BixTey films formed with and without CTAB.

(1)

As shown in Fig. 1(a), the potential became more negative upon addition of Bi3þ. If this process were due to the reduction of Bi3þ, we would have expected a potential shift in the opposite direction, i.e., to more positive values [17]. However, because the added CTAB and increase in Bi3þ concentration resulted in relatively fewer HTeOþ 2 ions available for deposition, the observed negative shift was reasonable. In addition, an increase in the current density was also observed. Fig. 1(b) shows the ratio of Te to Bi in BixTey films as a function of the Bi(NO3)3 concentration in electrolytes, obtained from a quantitative analysis using EDS. The Bi content in the electrodeposited films increased as the Bi3þ concentration in the electrolytes increased. This is attributed to the increased reduction of Bi3þ in electrodeposits. The deposited Bi content became saturated, which was expected since the LVS current density also

Y. Song et al. / Current Applied Physics 15 (2015) 261e264

became saturated. Near-stoichiometric Bi2Te3 thin films were obtained from electrolytes containing 7.5 mM Bi(NO3)3 with added CTAB. The SEM images in Fig. 2(a) show the surface morphologies and cross-sections of the electrodeposited BixTey thin films formed with and without CTAB. The BixTey thin film electrodeposited without CTAB had a rough surface and a porous structure. However, the film deposited with CTAB was smooth and more dense. Based on these results, we expect the film with CTAB to exhibit improved electrical and thermoelectric properties. Fig. 2(b) shows XRD patterns of BixTey thin films that were electrodeposited using different concentrations of Bi(NO3)3 in solutions with and without CTAB. The XRD peaks were indexed against standard references for Bi2Te3 and Au from JCPDS card No. 82-0358 and 04-0784, respectively. The main diffraction peak from the Bi2Te3 films deposited without CTAB can be indexed as a rhombohedral structure with the space group R3m (015). In Fig. 2(b), the XRD patterns of electrodeposited BixTey thin films using CTAB show decreasing intensity for the (015) main diffraction peak and increasing (110) peak intensity with increasing Bi(NO3)3 concentration. As this (110) preferred orientation was observed in films deposited by the underpotential deposition (UPD) mechanism [18], the (110) preferred orientation of Bi2Te3 thin films with CTAB might be attributed to UPD, which was also reflected in the shift in LSVs, as shown in Fig. 1(a). The films prepared using electrolytes with high concentrations of Bi ions had a strong (110) orientation, which was slightly enhanced with decreasing Te content, as shown in Fig. 1(b). This resulted in a denser surface and

263

Fig. 3. Electrical and thermoelectric properties of BixTey films deposited using different concentrations of Bi(NO3)3 in solutions with and without CTAB.

lower porosity compared with the Te-rich film (Fig. 3). In order to confirm the effects of the change in XRD peak orientation on the electrical and thermoelectric properties of the thin films, we measured the values of S and electrical resistivity (r) and calculated the power factor (P ¼ S2/r). Both S and r slightly decreased with the addition of CTAB, resulting in a maximal power factor caused by the change in crystal orientation. In addition, the carrier concentrations decreased slightly and the mobility increased by a factor of 1.5 by adding CTAB, which was due to the enhanced surface morphology, and which resulted in a decrease of resistivity. In addition, S was slightly improved (reduced) with the addition of CTAB, given the reduced carrier concentration and poorer (less negative) S value along the (110) orientation for BixTey without CTAB. The values of P for the Bi2Te3 films were still lower by two orders of magnitude than those of the bulk counterpart, which was mainly due to their poor crystal structure. Nanoindentation tests were then performed in order to understand the effects of the addition of a surfactant on the strength of films. Table 1 lists the values of hardness (H) and elastic modulus (Young's modulus, E), obtained at an indentation depth of less than 20% of the film thickness in order to minimize any substrate effects. The E and H values of Bi2Te3 films formed using the surfactant were significantly higher (300% and 2000%, respectively) compared to those of Bi2Te3 films formed without the surfactant. The Bi2Te3 films formed using the surfactant also exhibited improved mechanical characteristics versus those without CTAB, indicating a potential for greater machining reliability and favorable system integration. However, their elastic modulus is lower than that of bulk Bi2Te3 because of the smaller grain size in electrodeposits [19]. We performed post-deposition annealing to investigate its effects on crystal and thermoelectric properties. Fig. 4(a) shows XRD patterns of Bi2Te3 films formed using an electrolyte with 7.5 mM Bi(NO3)3 and CTAB and annealed above 200  C since annealing at lower temperatures is not effective for the improvement of thermoelectric properties [20]. Here, the crystal structure and preferred orientation were not changed, which might be attributable to the crystalline structure that had already been present in the as-

Table 1 Hardness and elastic modulus values of Bi2Te3 thin films deposited with and without CTAB. Fig. 2. (a) SEM top-view (scale bar ¼ 10 mm) and cross-sectional (scale bar ¼ 5 mm) images of Bi2T3 films formed with and without CTAB. (b) XRD patterns of BixTy films electrodeposited using different concentrations of Bi(NO3)3 in solutions with and without CTAB.

Bi2Te3 without CTAB Bi2Te3 with CTAB Bulk Bi2Te3

Hardness (GPa)

Elastic modulus (GPa)

0.4 ± 0.18 1.27 ± 0.11 1.26

16.8 ± 5.3 32.8 ± 1.9 41.80

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thermoelectric properties, which might be attributed to the constant level of crystallinity. Acknowledgments This research was mainly supported by the Global Frontier Program through the Global Frontier Hybrid Interface Materials (GFHIM) project of the National Research Foundation of Korea (NRF), funded by the Ministry of Science, ICT & Future Planning (2013M3A6B1078870). Support was partially provided by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) (Grant No. 20133030010890) and ISTK (Korea Research Council for Industrial Science and Technology) of Republic of Korea (Grant B551179-13-02-08). References

Fig. 4. (a) XRD patterns and (b) electrical and thermoelectric properties of Bi2T3 films formed using CTAB, after annealing at various temperatures.

deposited film. The electrical and thermoelectric properties of the annealed Bi2Te3 thin films are shown in Fig. 4(b). The power factor, however, exhibited no noticeable changes after annealing. Further work will be focused on maintaining the preferred growth orientation of the (015) plane. 4. Conclusions This study examined the effects of CTAB on the electrical, thermoelectric, and mechanical properties of electrodeposited BixTey thin films. The electrodeposited thin films formed using CTAB were smoother and denser than those without CTAB, resulting in improved mechanical properties. Nearly stoichiometric Bi2Te3 thin films were obtained from electrolytes containing 7.5 mM Bi(NO3)3, 10 mM TeO2, and 0.75 mM CTAB, which demonstrated a change in the preferred growth direction from (015) to (110) and the maximum power factor of 83.8 mW m K2. The power factor showed few changes when the concentration of Bi ions was changed in electrolytes containing CTAB. In addition, postdeposition annealing did not affect the electrical and

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